Despite the rapid growth in memory sizes of computers, storage in which data is placed in a computer requires careful conservation and recycling. Some programming languages support static allocation of storage space while others support dynamic allocation. Static allocation connotes that storage space is determined at compilation of the computer program. In the case of dynamic allocation, storage space is determined during execution of the programs, known as “run-time”. The area of dynamic memory reserved for storage space that is determined at run-time is a referred to as a “heap”.
Many programming languages place the responsibility with the programmer to explicitly allocate storage and explicitly dispose or free that storage when it is no longer needed. Thus, when the programmer has the responsibility of the allocating and recycling or reclaiming of storage in dynamic memory, the memory management is said to be manually performed or “explicit”.
An individually allocated piece of memory in the heap may be referred to herein as an “object”. The goal is to dispose of unused or dead objects by freeing such objects and returning the space associated with such objects to the “free list” or “free pool”. The “free list” or “free pool” is the set of unallocated usable spaces in the heap. However, with explicit allocation and deallocation, some objects cannot be deallocated or used because they have become “unreachable”. The space occupied by an unreachable object is said to have “leaked” away. In addition, explicit deallocation may result in “dangling references”. To illustrate, consider the following.
Data that is dynamically allocated in memory is usually accessible by the user program through the roots of the computation of the user program, or by following chains of pointers (“references”) from these roots. The roots of the computation of a user program are those locations that hold values that a user program can manipulate directly. Processor registers and the program stack are examples of roots of the computation. Thus, an object in the heap is said to be “live” if its address is held in a root, or there is a reference to it held in another live object in the heap. Otherwise, the object is said to be “dead”. An object becomes unreachable when the last reference to the object is destroyed before the object is deallocated. The object is no longer live if there are no references to it. In addition, such an object cannot be freed (deallocated) because it is unreachable, as there are no references leading to it. Objects that are neither live nor free cannot be used or recycled by explicit allocation and deallocation, and are called “garbage”. Thus, unreachable dead objects are “garbage”.
“Dangling references” occur when an object in the heap is deallocated while there are still references to the object. Dangling references can produce incorrect results. For example, if the memory location of the object that was deallocated while there were still references to it is reallocated to another of the user program's data structures, then the same memory location would represent two different objects.
Automatic dynamic memory management can obviate the problem of unreachable objects and dangling references. In one approach, automatic dynamic memory management involves assigning the responsibility for dynamic memory management to the program's run-time system. The programmer must still request dynamically allocated storage to be reserved but the programmer no longer needs to recycle the storage because the recycling is done automatically through a process called garbage collection. Garbage collection can recover unreachable objects by returning the space associated with such objects to the free list or free pool. In addition, garbage collection avoids dangling references by not deallocating any object while there remains references to it from other live objects.
Furthermore, garbage collection may be useful in correcting the problem of “fragmentation”. Fragmentation occurs in the course of allocating objects of different sizes in the heap. There are various strategies for choosing the optimum space in the heap to allocate to an object. However, such strategies usually result in areas or “fragments” of free pool in the heap. Sometimes the fragments are too small to be useful and are not contiguous and therefore, cannot be combined. For example, FIG. 1 illustrates a heap 100. The shaded areas 102, 104, 106, 108, 110 indicate storage space already in use in heap 100. The non-shaded areas, 101, 103, 105 indicate fragments of free pool in heap 100. Assume that none of the fragments of free pool are large enough to meet the current request for allocation of an object. Even though the free pool is not empty, an object cannot be allocated in the heap if none of the fragments are large enough. Thus, garbage collection can be employed to reclaim storage space not just when the “free-pool” is empty but also to automatically perform compaction of the heap storage space that is in use.
Garbage collection is one of many features associated with profiling a given user program such as an application program. “Profiling” describes the monitoring and tracing of events that occur during run-time. For example, a profiling tool may provide information on how much or how frequently dynamic memory is allocated by each portion of the executing application program. As will be explained in greater detail, some of the dynamic memory allocation information can be extracted from the object allocation process and the garbage collection process.
In one approach, profiling tools are specifically customized and instrumented for a specific platform. Typically, tool vendors specialize in providing tools that are only suitable for certain platforms. For example, different virtual machine vendors provide myriad Java™ virtual machine implementations. Java™ is an object-oriented language, the source code files of which are compiled into a format called bytecode. Bytecode can be executed on any machine on which Java run-time system software is installed. The Java™ run-time system is known as a Java ™ virtual machine, a self-contained operating environment that behaves as if it is a separate computer. The Java™ virtual machine translates the bytecode's set of instructions into the underlying computer processor's set of instructions. Thus, a Java™ application will execute in any Java™ virtual machine regardless of the hardware and software underlying the system. It would be highly desirable if users were given the choice of using any profiling tool that is available in the market without worrying if the tool is compatible with the user's particular implementation of Java™ virtual machine.
Based on the foregoing, it is desirable to develop a mechanism that supports a wide variety of profiling tools for garbage collected memory systems.